Physiology and chemistry of sex cell contact at fertilization shall be investigated in the mating type reaction of isogamous Chlamydomonas species, an agglutinative adhesion between sexually different gametes. Contact is based upon a complementarity between sex- and species specific glycoproteins. These mating type substances have been isolated and shall be analyzed in their chemical nature with the aim to characterize the proper contact sites and those features which cause the species specificity of the contact mechanism. The individual mating type substances will be characterized physicochemically (size, chromatographic and electrophoretic behavior) and chemically (elemental analysis, carbohydrate/protein ratio, sugar components and their mol ratio, amino acids and their mol ratio, presence of lipid, sulfate, and Ca). The isolated mating type substances shall be degraded in controlled manner by specific enzymes which do not destroy the proper contact site. The degradation products will be separated by chromatography and electrophoresis, determined in number and by size, and tested for their involvement in the contact function. The species specificity of sex cell contact shall be elaborated by comparing the contact systems of sexually compatible and incompatible species. The results are expected to give exact information on the molecular basis of sex cell contact and of cell to cell recognition, and will reflect upon the concepts of sexual isolation, species formation, and embryonal cell adhesion.